1. Field of the Invention
This invention relates to novel charged, random copolymers of acrylamide and acrylamido-substituted alkanoic acids or alkali metal salts thereof, such as 3-acrylamido-3-methylbutanoic acid or its sodium salt, as well as certain defined terpolymers thereof. More particularly, this invention relates to improved water flooding techniques for recovering petroleum from subterranean deposits which comprises adding minor amounts of the water soluble charged, random copolymers or terpolymers to the water flood as a mobility control agent.
2. Description of the Prior Art
Water-flooding as a method for secondary and tertiary recovery of oil is well-known in the art as a means for removing additional petroleum deposits from wells which are no longer pumping oil by routine methods. This method comprises injecting water into outlying wells under pressure and recovering oil from a central well, which oil is forced out of petroleum-bearing deposits by the water.
In the past, it has been discovered that this method is rendered more effective by the addition of various additives to the water, as for example, various surfactants and the like, but more particularly, materials which help control the mobility of the water through the rock strata, in order that it proceeds uniformly and does not pass solely through the more porous areas while by-passing the less porous rock. Thus, it is conventional to add thickening agents which increase the viscosity of the aqueous medium in order to overcome this problem. Frequently, this is done using aqueous media in various sequences, each containing different formulations of additives which enhance the desired secondary recovery and the sweep efficiency of the flooding medium.
In order to obtain mobility control in a flood, the displacing phase should have a mobility equal to or lower than the mobility of the oil. The addition of certain water-soluble polymers increases the viscosity of the water phase, and may in some instances reduce the permeability of the porous rock to water. In a heterogeneous reservoir of various porosities and fracture zones, the resistance to water flow can be significantly increased, the degree of fingering reduced, and a more uniform fluid front developed as the displacing phase moves through the petroleumbearing porous rock.
The complex physical and chemical interactions that polymer solutions encounter in the field are extremely demanding. The polymer must perform at relatively high temperatures for long periods of time under various conditions of pH, ionic strength (with various ion types), pressure, flow rate, and substrate heterogeneity. The polymer must resist shear degradation and must not absorb too strongly to the rock surface. (Preferential temporary absorption in larger pores in some instances could be beneficial for mobility control.) Multivalent cations present in the aqueous solution can lead to interactions of the charged polymers causing cross-linking, gel formation, precipitation, and pore clogging. The variety of pore sizes in reservoir rock can cause a chromatographic effect on the macromolecules. The larger molecules are excluded from smaller pores and therefore by-pass them in route to larger pores. Larger molecules thus tend to move ahead of smaller ones, and this occurs increasingly with longer distances. Polymer slug dispersion occurs thus resulting in loss of mobility control. Additionally, oil entrapped in small pores may be completely by-passed. (This portion of the rocks is referred to as "excluded" pore volume or "inaccessible" pore volume.)
Amongst the materials which have been added as viscosity-controlling agents for the aqueous medium, water soluble polymers have been preferred, including such materials as polyacrylamides, sulfonated polystyrenes, hydrolyzed polyacrylamides, copolymers of acrylamide with substituted acrylamides such as N-sulfomethyl acrylamide or 2-acrylamido-2-methylpropanesulfonate or the like. See, for example, U.S. Pat. Nos. 3,039,520; 3,679,000, or 3,804,173 which teach various polyacrylamides or copolymers of acrylamide and acrylamide derivatives as viscosity-controlling agents in oil recovery techniques. See also, U.S. Pat. No. 4,395,524 which teaches like copolymers for other industrial uses, as does Brit. Pat. No. 1,467,744, involving unsaturated cross-linked polyester polymers.
While many of these polymers have been found to be generally suitable as mobility agents, nevertheless many of them, amongst other concerns, have been found to be adversely affected by the presence of salts such as NaCl or CaCl.sub.2 which are frequently found in aqueous media used in water-flooding techniques. That is to say, it has been found that the viscosity of many water-soluble polymers, while effective for mobility control purposes in the absence of such salts, is substantially reduced in their presence. This is particularly so at the elevated temperatures found in many wells.
The prior art also teaches the use of acrylamide copolymers and the like which contain charged groups such as carboxylate groups in the polymer structure. See, for example, U.S. Pat. No. 3,679,000. The presence of such groups is desirable in theory because they should help minimize the adsorption of the polymer on the surface of the rock strata, thus avoiding depletion of the polymer in the aqueous medium and possible plugging of the formation. In practice, however, such charged polymers are very sensitive to salts in the aqueous media, particularly polyvalent salts containing divalent ions such as Ca.sup.++ and the like, which reduce the viscosity of the polymer solution significantly. The polymers of the present invention, however, which in their salt form also contain carboxylate groups, surprisingly maintain their desired viscosity in the presence of brines containing polyvalent, and particularly divalent, salts, while retaining the desirable characteristic of minimizing adsorption on the rock.
Therefore, it is an object of this invention to provide water-soluble, charged polymers as mobility control agents, i.e., as water thickeners for use in secondary and tertiary oil recovery techniques, which will maintain their desired viscosity in the presence of salts, particularly polyvalent salts, and at elevated temperatures, when added to the aqueous flooding medium in low concentrations.
It is a further object of this invention to provide an improved method for secondary and tertiary oil recovery by the use of said polymers.
These and other objects of this invention will appear from the following description.